This invention relates to the art of etching materials to form an array of similar structures, and more particularly, to etching materials so that each of the various structures has a particular characteristic that varies in a prescribed manner across the array.
A problem in the art of etching materials to forms structures is that the etchant, i.e., the substance which is causing the etching, may etch differently, e.g., at different rates or in a different direction, as a function of the type of material that is being etched and the location on the substrate which is being etched. The etch profile is a function of the conditions of the particular etch process employed. For example, using one particular type of chemistry employed for etching silicon with a gas plasma, photoresist nearer to an edge of a wafer is typically etched faster than photoresist nearer to the center of the wafer.
The variation of the etching across the substrate is called the etch profile. Furthermore, it may not be simply a variation in etch rate that is of concern, but instead it may be the variation in a ratio of the etching of different materials which are on the substrate, e.g., the ratio of the rate of etching of silicon to the rate of etching of photoresist, that is of concern. The variation of etch ratio across the substrate is a specific type of etch profile, called the etch ratio profile, which is really the result of combining two or more etch profiles.
The existence of a nonuniform etch profile, which is the general condition, is typically not desirable, because it causes the structures resulting from a uniform design, i.e., designed to be uniform in the presence of an assumed uniform etch profile, which is the only current design practice, to be not uniform, and hence to have different operating parameters. However, often uniform operating parameters are desired. For example, lens arrays are typically made by depositing uniformly sized disks of photoresist on silicon, flowing the photoresist into a lens shape, and then transferring the lens shape into the silicon by etching the silicon and photoresist simultaneously until the photoresist is completely etched. Disadvantageously, the nonuniform etch ratio profile for the etching of silicon to the rate of etching of photoresist will result in each lens having slightly different optical characteristics, e.g., different focal lengths, which is undesirable in optical communications applications that specify that each lens of the array must optically function substantially identically, because the lens-shaped photoresist is transferred into the silicon substrate at different rates over the wafer due to the nonuniformity of the etch profile of the ratio.
The same type of problem may also exist with other processing steps such as depositing, ion implantation, heat treatment, and irradiation, which each has their own associated processing profile which is analogous to an etch profile.
I have recognized that problems caused by a nonuniform processing profile can be avoided, in accordance with the principles of the invention, by altering the nature of the structures, i.e., some parameter thereof, in the area to be processed so as to compensate for the processing profile. More specifically, with regard to etching, I have recognized that problems caused by a nonuniform etch profile can be avoided, in accordance with the principles of the invention, by altering the mask employed in specifying the etch area so as to compensate for the etch profile.
In accordance with an aspect of the invention, with regard to etching, if desired, nonuniform parameters of interest of structures which result from a nonuniform etch profile during the etching of a mask in which all the structures were identical can be avoided by altering the mask employed in specifying the etch area so as to compensate for the etch profile. More specifically, the mask is changed in a manner that is inversely proportional to the etch profile for each particular structure characteristic that determines the parameter of interest for which uniformity is desired. Thus, for example, in the case of making lens arrays, the parameter of interest is the focal length, so long as each lens has a minimum diameter. To achieve a uniform focal length, the diameter and/or the height of the disks of photoresist is varied over the surface of the substrate. Varying the diameter is relatively easy to do, by varying the diameter of the disks on the mask. In order to achieve the same focal length for each lens in the array, assuming a uniform photoresist coating, the diameter of the disks has to be smaller in those regions where the etch ratio, e.g., the rate of etching of silicon as compared to the rate of etching of photoresist, is lower whereas the diameter of the disks has to be larger in those regions where the etch ratio is higher.
In accordance with another aspect of the invention, highly accurate intentionally nonuniform prescribed parameters can be achieved by employing a mask that not only has the desired nonuniform prescribed parameters as would result using an ideal uniform etch profile but has those parameters altered from their ideal desired values to compensate for the etching profile based on the location on the substrate at which the structure will be formed.